1. Field of the Invention
The present invention relates to a tightening tool, and particularly to a tightening tool having a silent clutch mechanism which is disengaged to stop the tightening operation upon tightening of a screw by a predetermined depth and which is kept silent during idle rotation even if a motor is continuously driven.
2. Description of the Prior Art
A conventional tightening tool having a silent clutch mechanism is disclosed in U.S. Pat. No. 4,655,103. FIG. 24 shows a view corresponding to FIG. 2a in this patent. The tightening tool disclosed in this patent has the following construction:
(1) The tool includes a housing, a motor, a clutch, a spindle 88 and a stopper sleeve 89.
(2) The motor, the clutch and the spindle 88 are accommodated within the housing.
(3) The stopper sleeve 89 is attached to the housing.
(4) A driver bit 90 engageable with a screw for rotation thereof is detachably mounted on the spindle 88.
(5) The driver bit 90 mounted on the spindle 88 is rotatable within the stopper sleeve 89.
(6) The spindle 88 is rotatably and axially slidably supported by the housing.
(7) The driver bit 90 is therefore slidably moved within the stopper sleeve 89 when the spindle 88 is slidably moved.
(8) The clutch mainly includes a first disc 81 rotatably driven by the motor, a second disc 84 facing to the first disc 81 and a third disc 87 fixedly mounted on one end of the spindle 88.
(9) The first disc 81 is rotatably and axially slidably supported by the housing.
(10) Clutch teeth 82 are formed on one end surface of the first disc 81.
(11) The second disc 84 is rotatably and axially slidably supported by the housing.
(12) Second clutch teeth 83 are formed on an upper end surface of the second disc 84 for engagement with the teeth 82.
(13) Recesses 85 and 86 are formed on the other end surface of the second disc 84 and on one end surface of the third disc 87 facing thereto, respectively. Each of the recesses 85 and 86 has a depth varying in a circumferential direction. More specifically, bottom surfaces 85a and 86b are connected to slant surfaces 85a and 86b each having a depth which becomes gradually shallower, respectively.
(14) The recesses 85 and 86 extend within a predetermined range in a circumferential direction and have vertical surfaces 85c and 86c at the boundary positions, respectively.
The conventional tool thus constructed is operated as follows:
(15) When the driver bit 90 is pressed on the screw, the driver bit 90, the spindle 88 and the third disc 87 are retracted together, so that the second disc 84 is also retracted.
(16) The clutch teeth 82 and the clutch teeth 83 are then engaged, and the recess 85 and the recess 86 contact each other by their bottoms 85a and 86a.
(17) A torque is transmitted from the motor to the driver bit 90 in this situation.
(18) When the second disc 84 is rotated by the first disc 81 and when the third disc 87 is subsequently rotated by the second disc 84, a contacting pressure is produced between surfaces of the recesses 85 and 86. Since each of the recesses 85 and 86 has a depth which varies in the circumferential direction, the contacting pressure forces the third disc 87 forwardly along the slant surfaces 85b and 86b.
(19) When the vertical surface 85c of the recess 85 and the vertical surface 86c of the recess 86 are brought to contact to each other, the force to move the third disc 87 forwardly is no more produced, and the tightening operation of the screw then proceeds in this situation. FIG. 24 shows such a situation.
(20) Upon tightening of the screw by a predetermined depth, the spindle 88 cannot be moved further, and the clutch teeth 82 and the second clutch teeth 83 are then disengaged from each other.
(21) When this occurs, the force of the second disc 84 to rotate the third disc 87 is no more produced, so that the second disc 84 is moved forwardly along the slant surfaces 85b and 86b of the recesses 85 and 86, resulting in that the recesses 85 and 86 contact each other at their bottoms 85a and 86a.
(22) A clearance corresponding to the depth (axial distance) of the slant surfaces 85b and 86b is consequently produced between the clutch teeth 82 and the second clutch teeth 83, and therefore, the clutch is silently idly driven without causing the teeth 82 and 83 to abut on each other.
In such a conventional tightening tool, the transmission of torque is performed through engagement of clutch teeth 82 and 83 with each other. Therefore, it involves problems in the durability of the clutch teeth and the remarkable damage and wearing of the clutch teeth. Particularly, at the beginning of the tightening operation, when the housing of the tool is pressed on a work to be tightened, the spindle 88 is retracted together with the driver bit 90 by such a pressing force, and the teeth 82 of the first disc 81 which is rotatably driven abruptly abut on the clutch teeth 83 of the second disc 84 to transmit rotation thereto. Therefore, the abutment of the surfaces of the teeth 82 and the surfaces of the teeth 83 on each other produces the problem in the durability and in the wearing.
Additionally, the conventional tightening tool is constructed such that the clutch teeth 82 on the fixed side and the clutch teeth 83 on the movable side are positively entirely disengaged from each other by movement of the second disc 84. Therefore, an additional tightening operation for further tightening the previously tightened screw cannot be performed even if the amount of tightening of the screw was insufficient as compared with a desired amount because of the inappropriate adjustment of the stopper sleeve or because of the operation at a narrow working place or on an unreliable foothold.